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Unformatted text preview: PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 Digestive Motility
Outline - What is the fxn of GI motility? - Describe the physiology of GI smooth muscle - How does food move in the GI tract? - What happens during vomiting? Textbook reading: 693-4, 709, 716, 718-9 (5E); 682-684, 699-700, 707-709, 710 (4E) Recall: Functions of digestive system Recall: Fxns of digestive system! ! Recall: layers of GI tract Recall: layers of the GI tract Mucosa Submucosa
CIRCULATION Muscularis externa Serosa CONTROL Recall: control of GI function
Modified from Figure 21.2 Recall: control of GI fxn Figure from Martini (2006) Fundamentals of Anatomy and Physiology ~ Fig 21.3 What is the fxn of GI motility? Figure 21.11 - Moves food from the mouth to the anus - Mechanically mixes food for max exposure to digestive enzymes & epithelium for absorption - Motility determined by properties of smooth muscle - Generally controlled by the enteric nervous system (ENS) 1 of 7 PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 Describe the physiology of GI smooth muscle - Most of GI tract composed of single-unit smooth muscle - Groups of cells electrically cxted by gap jxns - AP travels in all directions - Create contracting segments - Innervated by the ANS signals transmitted simultaneously thru a single unit Describe the physiology of GI smooth muscle single unit Figure 12.25 Activity in smooth muscle cells = cyclic - Slow waves - Slow undulating changes in potential - Freqcy varies in GI tract - Do not cause contraction (does not reach threshold) - Spike potentials - Freqcy dependent on slow wave activity - APs occur when resting membrane potential reaches threshold - Becomes (+) = contraction of smooth muscle - Affected by stretch, ACh, parasympathetic activity - Hyperpolarization: NE, sympathetic stimulation (no food present) Modulation of membrane potential - Force & duration of muscle contraction arein smooth muscle cells & freqcy of APs Activity directly related to amplitude
Slow waves slow undulating changes h do not cause contraction - cells connected by gap junctions - action potential travels in all directions Spike potentials occur when membrane potential reaches th h ld h threshold frequency p dependent on the Specialized smooth muscle cells have pacemaker activity slow wave - Originate from interstitial cells of Cajal - Electrical pacemakers for smooth muscles of intestine (rate & rhythm) - Generate slow wave potentials & APs - Lie at interface btwn nerve fibers & (non-pacemaker) smooth muscle cells - Commc btwn ENS & smooth muscles - Electrically coupled to smooth muscle cells via gap jxns Specialized smooth muscle cells have p pacemaker activity y
Interstitial cells of Cajal 21.4 Figure Generate slow wave potentials and action potentials Electrically coupled to smooth muscle cells via gap junctions Lie at interface between nerve fibers and smooth muscle cells 2 of 7 PSL302: Lecture 65, by French! Comparison of pacemaker activity in h Mon. Mar. 28, 2011 Comparison of pacemaker activity in heart vs. GI tract (a) Heart pacemaker cells - Freqcy ~80/min - Pacemaker potential initiates APs (b) Interstitial cells of Cajal - Freqcy varies throughout GI tract (e.g. 3/min stomach to 12/min duodenum) = much slower than myocardial cells - Potentials do not reach threshold per cycle - Depolarization phase of AP due to Ca2+ entry (like heart) - Ca2+ entry also causes contraction in muscle fiber (graded according to [Ca2+] - Slow wave potential initiates APs - duration of slow wave = APs & contraction force contraction duration - Amplitude & duration can be modulated by neurotransmitters/hormones/paracrines Modulation of membrane potential by the ANS Heart pacem frequency a Interstitial c frequency v tract (e.g. 3/m 12/min duod can be mod depolarizat potential due Modulation cont'd - Parasympathetic - Nerve cxns: long preganglionic fibres & short postganglionic neurons (both ACh) - Vagus innervates esophagus, stomach, S & L intestine - Pelvic nerve (sacral division) innervates L intestine, rectum & anus - Effects: gut muscle activity ! ! relaxes sphincters - Sympathetic - Nerve cxns: shorter preganglionic fibres (ACh) & longer postganglionic fibre (NE) - Postganglionic neurons innervate all portions of the GI tract - Effects: gut mvts, constricts sphincters - Afferent sensory fibres http://ethesis.helsinki.fi/julkaisut/laa/biola/vk/peuhkuri/chap2.html - Either or gut activity or secretion - Respond to irritation (of Interdigestive state: Migrating myoelectric complex mucosa), distension & chemical stimulation 3 - Hormones: either or activity phases How does food move in the GI tract?
Upper esophageal sphincter Lower esophageal sphincter Parasympat increase gu activity relaxes sph Sympathetic inhibit gut m constricts s Hormones either increa decrease act phase 3: rhythmic contraction of circular muscle that propagates along length of intestine every 80-110 min How does food move in the GI tract?
Phase 2 Phase 3 Phase1 Swallowing Peristalsis Pyloric valve Ileocecal valve Internal anal sphincter External anal sphincter Grinding and peristalsis Peristalsis and segmentation Segmentation and mass movement Defecation
Figure 21.1 Quigley and Thompson (1998) Braz J Med Biol Res 31:889 Interdigestive state: migrating myoelectric complex 3 of 7 PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 Interdigestive state: migrating myoelectric complex - Muscle contractions in GI tract occur in 3 general phases - Phase 3: interdigestive state (not eating) - Migrating myoelectric complex: rhythmic contraction of circular muscle that propagates from stomach to L intestine, every 80-110 min - Housekeeping: sweeps food remnants & bacteria out of upper GI tract & into L intestine - During and following a meal: muscle contractions fall into one of two patterns - Peristalsis: progressive waves of contraction that move from one section of GI tract to next - Circular muscles contract behind bolus push it fwd into receiving segment - Influenced by hormones, paracrine signals & ANS - Segmentation: short segments of intestine alternately contract and relax - In contracting segments, circular muscles contract & longitudinal muscles relax - Contractions may occur randomly along intestine, or at regular intervals - Alternating contractions church intestinal contents mix & keep in contact w/ absorptive epithelium - Sequential contractions in oral-to-aboral direction digesting material propelled short distances Example: motillin - Hormone released from duodenum during interdigestive state - Acts on myenteric neurons & smooth muscle cells - Help trigger migrating myoelectric complex Swallowing (deglutition): Reflex action that pushes a bolus into esophagus - Stimulus: pressure created when tongue pushes bolus against soft palate & back of mouth - Sensory input swallowing center in medulla = trigger reflex Swallowing (Deglutition) Swallowing -- Volun
Figure 21.24 Swallowing: voluntary - Food CN IX, X swallowing center in medulla/lower pons See animation: Interactive Physiology, Motility, page 4 CN V, IX, X, XII - Tongue pulled up against hard palate = bolus into pharynx - Respiration inhibited Food swallowin lower pon tong again resp p 4 of 7 PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 Swallowing -- Involuntary
Food in pharynx CN X ! : involuntary (vagus) swallowing - Food in pharynx CN X (vagus) swallowing center center CN V, VII, IX, X, XII CN V VII IX, X, XII V, VII, IX X - Soft palate moves up pharynx & upper esophageal sphincter relax = food enters esophagus upward movement of soft p - Closing of larynx by epiglottis & vocal cords palate closing of epiglottis - Base of tongue to push food into esophagus Esophagus - Food vagus swallowing center vagus myenteric plexus - Peristalsis & relaxation of lower esophageal sphincter - LES not a true sphincter but a region of high muscle tension = barrier Peristalsis - Stimulated by distention - Progressive waves of contraction food thru GI tract - Intrinsic regulation of peristalsis - Contraction stimulated by ACh & substance P - Intrinsic regulation of peristalsis Relaxation via inhibitory neurons that release NO, vasoactive intestinal peptide & ATP base of tongue to push food into esophagus Peristalsis progres contracti propel tract stimula See video 1 at http://www.n tents/pt1/full/ Figure from Martini (2006) Fundamentals of Stomach relaxes inhibitory neuronsStomach undergoes waves of peristalsis to to accommodate food Anatomy and Physiology ~ Fig 21.5 Relaxation via that release chyme and empty Stomach relaxes to accommodate food both mix - Vagovagal reflex: stomach to and ATP stomach NO, NO vasoactive intestinal peptide brain to
Figure from: http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html Contraction stimulated by Ach and substance P Vagovagal reflex stomach to brain stomach g g - Stomach undergoestowaves of peristalsis to both mix chyme & empty - Upper stomach initially relaxed - Proximal of stomach contracts = pushes chyme into distal stomach - Pyloric sphincter closed churning & grinding & mixing - Pylorus relaxes wave moves down to pylorusSmall intestine: Segmentation - Subsequent wave moves chyme into distal stomach - Strong antral contraction enhances retropulsion & grinding localized concentric
See animation at: Interactive Physiology, Motility, page 6, also video at: http://www.youtube.com/watch?v=o18UycWRsaA&feature=related
Figure from Interactive Physiology, Pearson Figure from Interactive Physiology, Pearson Small intestine: segmentation along the intestine - Peristalsis & segmentation to propel food' circular muscles contract/ longitudinal relax - Slow mvt for digestion/absorption - Localized concentric contractions at intervals along intestine - Circular muscles contract - Longitudinal muscles relax contractions at intervals t ti ti t l Figure 21.5 5 of 7 PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 Ileocecal (IC) valve controls emptying of small intestine and prevents back flow p Ileocecal (IC) valve controls emptying of small intestine & prevents back flow - By end of ileum, only 1.5L of unabsorbed chyme remains - Colon absorbs most of this volume = 0.1L water excreted in feces - Ileocecal valve: tonically contracted region of muscularis - Narrows opening btwn ileum & cecum - Opened by distension of ileum & by gastroileal reflex - Distension of ileum: relaxes each time a peristaltic opened by distension of ileum and by the gastroileal reflex wave reaches it stomach - Gastroileal reflex: relaxes when food leaves the closed by distension of the colon - Closed by distension of colon Large intestine: mass movement - 1-3 per day, usually after a meal - Mvt of material up ascending colon, across thru transverse colon, & down thru descending colon & sigmoid colon - Rectum is short terminal section of L intestine - Separated from ext envmt by anus (opening closed by two sphincters, an internal smooth muscle sphincter & an external skeletal muscle sphincter) - Waves of contraction moves content larger distances - Slow: 5-10 cm/h - Chyme is mixed by segmental contractions - Fwd contraction miminal and depends primarily on mass mvt (occur 3-4 times a day) - diameter of segment of colon & sends substantial bolus fwd - Assoc w/ eating & distension of stomach thru gastrocolic reflex - Responsible for sudden distension of rectum = trigger defecation - Controlled by ENS, parasympathetic (+) & sympathetic (-) NS Defec Defecation mass movement in colon pushes material into the rectu Defecation reflex - Spinal reflex: triggered by distension - Removes undigested feces from body - Mass mvt in colon pushes material into the rectum - Rectal distension ENS relaxation of internal anal sphincter - Peristalsis push material twd anus - Parasympathetic stimulation enhances peristalsis internal anal sphincter - Voluntary relaxation of external sphincter, if external anal sphincter situation mass movement in colon is appropriate (voluntary) - Aided by pushes material into the rectum voluntary contraction of abdominal muscles & forced expiratory mvts against rectal distension ENS peristalsis and relaxation of internal closed glottis rectal distension ENS anal sphincter parasympathetic stimulation voluntary relaxation of exte contraction of abdominal m anal sphincter parasympathetic stimulation enhances peristalsis voluntary relaxation of external sphincter contraction of abdominal muscles, expulsion of feces 6 of 7 PSL302: Lecture 65, by French! Mon. Mar. 28, 2011 - Subject to emotional influence - Stress may intestinal motility cause psychological diarrhea in some individuals - But it may also motility cause constipation in others - When feces voluntarily retained in colon (either thru consciously ignoring a defecation reflex or thru motility), continued water reabsorption = hard, dry feces hard to expel Review - What might happen if the ileocecal valve was damaged and could not close properly? (a) Absorption of nutrients and water would be reduced (b) The individual would become constipated (c) Digestion of protein in the stomach would be reduced (d) Peristalsis in the esophagus would be increased What happens during vomiting? - Vomiting (emesis): forceful expulsion of gastric & duodenal contents from mouth - Protective reflex that removes toxic materials from GI tract before absorbed - BUT excessive or prolonged vomiting loss of gastric acid = metabolic alkalosis - Stimulation of sensory receptors & nausea vomiting center in medulla - Vagus - Enhances salivation - Relaxes esophagus, LES, body contracts pylorus - Spinal nerves - Inspiration: respiration inhibited - Trachea & nasopharynx closed off by epiglottis & soft palate - Contract abdominal muscles ( intra-abdominal presure) - Phrenic nerve - Diaphragm descends - Stomach - Begin in small intestine & move upwds = undergoes reverse peristalsis - Relaxes = pressure forces gastric & intestinal contents out 7 of 7 ...
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